1. Field of the Invention
The present invention relates to optical wavelength control method and apparatus for controlling an optical wavelength of a light source with a uncertain and unstable central wavelength, by pulling it into a specific optical wavelength or a reference optical wavelength defined with respect to an optical wavelength of an external input light.
2. Description of the Background Art
A laser light source containing a semiconductor laser has a central wavelength which is uncertain and unstable at nanometer order (approximately 125 GHz for optical frequencies in 1.55 μm band) due to its operation conditions such as injected current, pumping light power, environmental temperature, etc. A control technique for discriminating and stabilizing an optical wavelength of light outputted from such a light source, or a control technique for controlling an optical wavelength of light outputted from such a light source to follow an optical wavelength of an externally entered light, is therefore indispensable in systems (which are to be generically referred to as optical communication systems hereafter) such as an ultra high speed optical transmission system, a wavelength division multiplexed optical transmission system, an optical measurement device, etc.
As a conventional control technique for discriminating and stabilizing an optical wavelength of control target light outputted from a laser light source, there is a method using a cell filled with a gas of ammonia or acetylene that has absorption lines at specific optical wavelengths (optical frequencies). This is a method in which the optical wavelength of the light source is controlled by irradiating the control target light onto that gas cell and observing a light absorption rate at the gas cell. For example, there is a method for controlling the optical wavelength of the control target light which is already in practical use, in which a part of the control target light outputted from a semiconductor laser light source is entered into an acetylene gas cell that has sharp absorption lines at 1.51 to 1.55 μm range, and the semiconductor laser is adjusted such that a rate of transmitted light always stays minimum.
However, in this conventional method using a gas cell, the absorption line optical frequency of the gas is sensitive to the filling atmospheric pressure, the environmental temperature, etc., so that this method has been unable to sufficiently meet the application conditions of the optical communication systems for which a high reliability is required.
Also, in this conventional method, the optical frequency pull-in width is approximately several hundred MHz to several GHz and restricted to a vicinity of the absorption line optical frequency of the gas cell, so that it is often unable to meet various practical requirements. Namely, this conventional method cannot be considered as having a sufficient optical frequency pull-in width for controlling the optical wavelength of the laser light source with a central wavelength which is uncertain and unstable at nanometer order (approximately 125 GHz for optical frequencies in 1.55 μm band).
From a viewpoint of the optical wavelength control for a laser light source, relevant prior art techniques can be classified into the following three categories.
(1) A laser light source optical frequency stabilization technique using a gas cell or an optical filter as optical frequency discrimination means.
(2) A laser light source optical frequency control technique using a diffraction grating as optical frequency discrimination means.
(3) An optical phase synchronization technique using optical PLL (Phase-Locked Loop).
The technique (1) realizes the stabilization control of the laser oscillation frequency by setting the laser light source optical frequencies in a vicinity of the absorption wavelengths of the gas cell or the transmission wavelengths of the optical filter and carrying out the optical frequency discrimination. However, in this technique (1), the absorption wavelengths of the gas cell or the transmission wavelengths of the optical filter are distributed in an optical frequency space discretely, so that it is difficult in principle to realize the laser light source optical frequency stabilization by setting the optical frequencies continuously (see Japanese Patent Application 2-201987 (1990), for example).
The technique (2) realizes the laser oscillation optical frequency control in a similar way as in the technique (1), by using the Bragg diffraction grating, AWG (Array Waveguide) filter, etc., as the optical frequency discrimination means. As in the case of the technique (1), in this technique (2), it is difficult in principle to realize the laser light source optical frequency stabilization by setting the optical frequencies continuously. In addition, as a DC driven differential detector is to be utilized as means for detecting a state of the optical frequency discrimination, so that there is a practical problem that it is difficult to remove DC drift components of a control circuit (see Japanese Patent Application 9-199779 (1997), for example).
Moreover, both the technique (1) and the technique (2) do not have a sufficient pull-in width with respect to a semiconductor laser light source with a large optical frequency variation in which the oscillation optical frequency can vary by 100 GHz or more, so that these techniques lack the reliability.
The technique (3) can realize the optical frequency pull-in control continuously with respect to an offset frequency defined with reference to a light source with a high optical frequency stability. However, in this technique (3), the pull-in range of the optical PLL circuit is limited to about 100 MHz at best, which is three orders of magnitude smaller than the optical frequency variation width of the semiconductor laser, so that it must be used in combination with the other optical frequency stabilization technique.